1. Field of the Invention
This invention relates generally to industrial, mining and construction tools, and more specifically to improvements in rotary drag bits and the like for boring and drilling operations and to methods for rock mining using such tools.
As used in the following disclosure and claims, the term "polycrystalline diamond" and its abbreviation "PCD" refer to a material formed of individual diamond crystals fused or sintered by intercrystalline bonding under high pressure and temperature into a predetermined layer or shape. The PCD material is usually permanently bonded to a substrate of tungsten carbide in a cobalt binder or like carbide matrix, also known in the art as "precemented carbide". Also, as used herein, the term "high density ceramic" or its abbreviation "HDC" refers to a mining tool having an insert embodying a PCD layer.
2. Prior Art
In the past rotary drilling and coring tools, as used in mining and construction, have been constructed with hardened drill bit cutting heads, and traditionally with sintered carbide inserts to prolong the operative life of the tool. Typical cutting tools may use a single or continuous cutting surface or edge, but most frequently employ a plurality of discrete cutting elements or coring bits either sequentially and angularly arranged on a rotary bit or auger of some type. The class of heavy duty cutting tools, to which the present invention pertains, involve industrial mining and construction equipment of rotary drag type. This class generally includes rotary roof bits, longwall radial bits, auger drill bits, undercutter bits, core barrel bits, face drill bits, and two-wing, three-wing and four-wing rotary drag bits--all of which are readily identifiable to those in the mining field.
A principal problem encountered in all of these prior art tools has been the rapid wear and high cost of replacement along with machine down-time. Such rapid tool wear and breakage, in part due to higher speed equipment and heavier frictional forces and tensile stress, has led toward tool redesign with some larger carbide insert or drilling tip configurations--which in some applications has resulted in higher dust levels and increased potential ignition dangers contrary to mining safety regulations.
It is believed that a primary and inherent contributing factor in tool wear and breakage heretofore has been the conventional design configuration of such tool bits, together with traditional mining methods using combinations of heavy tool thrust and fast rotational speeds along with low pressure delivery of flushing fluids. Typically, substantially all prior tools have been constructed with a positive to zero rake angle thereby presenting a leading cutting edge or high entry point and trailing face that operates with a plow-type action and is subjected to high-point shear forces and tensile stress and drag. The typical positive angularity of cutting edge/face design produces rapid wear and failure, even in the tougher bits using tungsten carbide inserts and the like.
More recently, some substantial advances have been made in harder, tougher compositions for bit inserts. U.S. Pat. Nos. 4,525,178; 4,570,726; 4,604,106 and 4,694,918 disclose some of the basic underlying technology pertaining to such compositions and methods of making PCD materials proposed for use in various oil field drilling and mining operations as well as other machining operations. In particular, U.S. Pat. No. 4,570,726 discloses special insert shapes for coring-type rotary drill bits and suggests a tool having a working surface positioned at a slight negative angle from the perpendicular with respect to the material contacted. In fact, the '726 patent teaches away from the planar-type of working surfaces of both the prior art and the present invention, and discloses specially designed curved face insert configurations for obviating the backup or build-up of loosened material against the working surface. Another patent--U.S. Pat. No. 4,303,136 shows another coring tool having a series of drag bits with diamond surface layers carried on tungsten carbide bodies at a substantial negative rake angle, but this patent relates primarily to the orientation of the working face to hydraulic fluid passages for carrying off the loosened material.
Despite the transition toward increased use of PCD materials in rotary drag bit tools, traditional mining methods have continued to be employed. Thus, a typical prior method for obtaining optimum results in rock boring with carbide insert tools uses a fast rotational speed of about 500 to 1000 rpm with a heavy thrust of about 5000 to 13,000 psi, and wet carbide drilling conventionally uses a low water delivery pressure in the range of 60-150 psi.